1. Field of the Invention
The present invention relates to a non-grounded type ultrahigh frequency antenna used for cellular phones, etc.
2. Prior Art
Some conventional antennas are used as non-grounded type antennas by setting the antenna length to be .lambda./2 so that the antenna is caused to be parallel resonant within the working frequency band and raising the impedance of a feeder so as to reduce unbalanced earth currents. In these conventional antennas, it is impossible to raise the antenna impedance at resonance above a predetermined level, which is approximately 200 to 500 ohm. Accordingly, complete non-grounding is not obtainable, and the antennas is not broad-band. For this reason, the conventional antennas, with special exceptions, can only be used for portable wireless telegraphs in which the inside radio set and antenna are directly connected.
Brown antennas as illustrated in FIG. 10 are one of the non-grounded type antennas known in the prior art. This antenna has several (for example four) ground wires each having a length .lambda./4 and being attached to a connecting point between the antenna and a coaxial feeder. The impedance of the grounding side is set higher to eliminate unbalanced earth currents. In this way, the Brown antenna is usable as a non-grounded type antenna.
Another example of prior art non-grounded type antenna is a wave-trap type antenna as shown in FIG. 11. In this antenna, a coaxial trapping circuit, which is of .lambda./4 length and connected to a coaxial outer wire, is provided between the antenna and a coaxial feeder so that the antenna is usable as a non-grounded type antenna.
In the Brown antenna, since a large space is required for its base section, the appearance of the entire antenna tends to be unpleasant and poor. On the other hand, since the wave-trap type antenna has a narrow band, two or three steps of antenna rods are usually required so that the antenna is usable as a broad band antenna. Furthermore, because a relatively large diameter cylinder of .lambda./4 length is mounted at the base of the wave-trap antenna, the overall antenna length becomes fairly long. In addition, because of such a large-diameter cylinder, the antenna does not have the appearance of being light weight.